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Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
Central Oregon Agric. Res. Ctr., 850 NW Dogwood Lane, Madras OR 97741
* Corresponding author (rshibles{at}iastate.edu).
A low concentration of the S-containing amino acids, cysteine and methionine, often limits the nutritive value of soybean [Glycine max (L.) Merr.] seed protein. On a whole-plant level, the localization and duration of cysteine and methionine synthesis during seed filling in soybean have not been reported previously. The objective of this study was to characterize the localization and duration of ATP sulfurylase activity in reproductive soybean. ATP sulfurylase is the first enzyme in the S-reduction pathway. Samples of soybean (cv. Kenwood 94) root, leaf, stem, pod wall, and seed tissues on a whole-plant basis were taken at beginning seed filling (approximately R5.5) from two field sites in Iowa in 1997. In addition, samples of leaf, pod, and seed tissue from the same nodal position (Node 11) were collected weekly from R5 to R8 (full maturity) at one of the sites. At R5.5 almost half of whole-shoot ATP sulfurylase activity was located in seed tissue. Leaf tissue contributed approximately 30 to 40% of the shoot ATP sulfurylase activity, and stems and pod wall contributed approximately 10% each. The profile of specific activity (fresh weight basis) for leaf, pod, and seed tissue at node 11 over time showed that activity of leaf tissue steadily declined during seed tilfing, whereas that of seed tissue increased initially with seed expansion and remained high until physiological maturity. Thus, in terms of both specific activity, and sustained duration of activity, seed tissue was the dominant site of ATP sulfurylase activity during seed filling. It appears, then, that most of the S-containing amino acids synthesized during reproductive growth may be synthesized in the seed itself, rather than being imported from other tissues.
Received for publication November 28, 1997.
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